The present invention relates to aqueous binder dispersions based on silane-modified polymeric binders and inorganic nanoparticles, a process for the preparation thereof and the use thereof for the preparation of high quality coatings, in particular clear lacquers.
Nanoparticles in polymeric coatings can improve properties such as scratch resistance, UV protection or conductivity in a targeted manner. Control of the surface modification and dispersing of the nanoparticles determines the required transparent appearance of the coatings and properties thereof.
Various approaches have been pursued in the past for introduction of the nanoparticles into coating composition formulations. In this context, the particles can be mixed directly into the resin or curing agent component or into the coating composition ready for application. In aqueous systems there is the possibility of dispersing the particles in the aqueous phase. The in situ preparation of the particles in one of the binder components and adaptation of the surface to either the resin or the curing agent component have furthermore been described.
From the practical point of view, it is advantageous to disperse the nanoparticles as stable masterbatches in one of the components, so that a long-term storage stability and a simple ease of handling in the formulation of lacquers is ensured. In the end use, the nanoparticles must likewise be readily dispersible in a finely divided manner, so that advantageous properties such as transparency, scratch resistance or conductivity result.
In practice, the nanoparticles are conventionally dispersed into the resin component, into the aqueous phase or into the finished mixture of curing agent and resin shortly before curing. As a rule, for this it is necessary to adapt the surface of the nanoparticles to the specific matrix of the coating composition or of the adhesive. The disadvantage of simple mixing in of modified nanoparticles is the dependency of the stability on the complete formulation, i.e. on all the formulation constituents. Variation of one parameter can lead here to demixing (Pilotek, Steffen; Tabellion, Frank (2005), European Coatings Journal, 4, 170 et seq.).
It is known from the prior art that coating compositions can be prepared with silane-modified nanoparticles. For example, WO-A 02/24756 discloses coating compositions based on suspension polymers and silane-modified nanoparticles. However, the polymers described there are suspension and emulsion polymers. Suspension polymerization is carried out in an aqueous phase, like emulsion polymerization. Beads of solid, which are filtered off from the aqueous phase, form the end product of the suspension.
WO-A 2006/008120 describes aqueous dispersions of polymeric and/or oligomeric organic binders and inorganic nanoparticles. The nanoparticles are surface-modified by addition of silane-functional compounds. The disadvantage here, however, is that the gloss and haze of the resulting coatings do not meet the high requirements of automobile clear lacquers.
WO-A 03/095532 discloses aqueous dispersions of hydrophilized polymers or oligomers which contain surface-modified inorganic nanoparticles and at least one amphiphile. Possible hydrophilic polymers are emulsion polymers, which are prepared in an aqueous phase. The use of the amphiphiles, which are necessary according to this teaching, is a disadvantage since these low molecular weight alcohols must be included as solvents in the VOC balance (volatile organic compounds). Furthermore, these are highly reactive mono- or polyol compounds which co-react, for example, with polyisocyanates during crosslinking of the binders and lead to a reduced functionality up to chain termination. The network build-up of the resulting lacquers impaired in this way leads to a reduction in resistances.
It has now been found, surprisingly, that aqueous copolymers which are modified with a certain class of silanes are suitable, in combination with inorganic nanoparticles, for the production of coatings having a significantly improved scratch resistance with excellent gloss and very low haze (cloudiness).
The object of the present invention was thus to provide high quality coating compositions, in particular as automobile clear lacquers, which have an optimum gloss and haze and show an improved scratch resistance. The dispersions should furthermore be sufficiently stable to storage.